Islamic State jihadists, emboldened by a string of battlefield victories, advanced Thursday to the gates of the Syrian city of Hasakeh after intense fighting with regime troops.

Despite nine months of U.S.-led airstrikes, Islamic State (ISIS) militants have made new territorial gains in recent weeks, seizing areas that include the ancient Syrian city of Palmyra and the capital of Iraq's vast Anbar province.

Now ISIS "is 500 meters (550 yards) away from the entrance of Hasakeh, after fierce clashes against regime forces south of the city," said Rami Abdel Rahman, director of the Syrian Observatory for Human Rights, a Britain-based monitoring group.

He said ISIS had seized all military posts in that area of northeastern Syria, including an unfinished prison building and a power plant, after at least six suicide bombers struck on Wednesday.

Regime helicopters dropped barrel bombs on jihadist positions, he said.

If ISIS seizes Hasakeh, it would be the second Syrian provincial capital to fall under its control, after Raqa, the group's bastion in northern Syria.

It would also be the third provincial capital lost by the regime. The northwestern city of Idlib was seized by rebels in March.

Iran and Iraq have deployed thousands of fighters in Syria in recent weeks to help the regime defend Damascus and its surroundings, a security source told AFP on Wednesday.

"Around 7,000 Iranian and Iraqi fighters have arrived in Syria over the past few weeks and their first priority is the defense of the capital. The larger contingent is Iraqi," the source said on condition of anonymity.

Iran is a key ally of the Syrian government, and it has provided Damascus with financial and military support.

But in recent months, the Syrian government has lost territory in several parts of the country to both ISIS and rebel groups, including al-Qaida affiliate al-Nusra Front.

Fighting on Wednesday around Hasakeh left 27 regime fighters and 26 jihadists dead, according to the Observatory, which relies on a network of sources on the ground.

The Al-Watan newspaper, which is close to the government, said ISIS had begun a "violent attack on Hasakeh," control of which is divided between regime troops and Kurdish fighters.

It criticized Kurdish forces for failing to support regime troops, even though they have weapons "given to them by the Syrian state."

Despite the assault, Hasakeh provincial governor Mohammad Zaal al-Ali insisted the city was "completely safe" in remarks quoted by the official news agency SANA.

ISIS began its assault on Hasakeh on May 30 and launched a simultaneous offensive against rebels in northern province of Aleppo.

The attacks follow the jihadists' capture last month of Palmyra and its 2,000-year-old UNESCO World Heritage site in central Syria. It has also expanded its control along the Syrian-Iraqi frontier.

In neighboring Iraq, ISIS fighters last month seized Ramadi, the capital of the country's largest province, Anbar, in a major setback to the government.

ISIS militants used an unprecedented wave of suicide truck bomb attacks to seize Ramadi in a three-day assault.

The U.S.-led coalition launched an air raid Wednesday that leveled one of ISIS' largest car bomb factories, causing a massive explosion heard far away, officials said.

Iraqi officials said the strike in the town of Hawijah destroyed a factory packed with tanks, Humvees, and large quantities of explosives.

In a television interview late Wednesday, the head of al-Nusra Front described ISIS' self-proclaimed caliphate as "illegitimate."

His face hidden from view by a black scarf, Abu Mohamed al-Jolani said the ISIS caliphate was "not based on Islamic law," and said the rivalry between the two jihadist groups was not likely to end soon.

"There is no indication of a solution between us and them at this time. We hope that they repent and return to the Sunni people," Jolani said.

In Turkey, Syria's opposition National Coalition met with U.N. peace envoy Staffan de Mistura on Thursday, as part of "ongoing discussions about a political solution," a coalition spokesman said.

More than 220,000 people have been killed in Syria since anti-government protests broke out in March 2011 and escalated into a full-blown civil war that brought jihadists streaming into the country.

U.S. Deputy Secretary of State Antony Blinken said Wednesday that 10,000 ISIS members had been killed since the start of the U.S.-led air campaign in Iraq and Syria.